1. Introduction
This invention relates to the purification of novolak resins. More particularly, this invention relates to the removal of various ions from resins used in the manufacture of photoresists.
2. Description of the Prior Art
Phenolic resins such as novolak resins and polyvinyl phenol resins are known and used as binders in many coating compositions. A major use of such resins is in the formulation of photoresist compositions. Novolak resins are disclosed as photoresist binders generally in U.S. Pat. No. 4,404,272. Polyvinyl phenol resins is disclosed as photoresist binders in U.S. Pat. No. 3,869,292. The manufacture of both resins may be conducted in the presence of a strong acid during the polymerization reaction.
The formation of phenolic novolak resins by condensation of a phenol with an aldehyde is well known in the art and described in numerous publications including the Kirk Othmer Encyclopedia of Chemical Technology, Volume 15, pages 176 to 208, 1968, incorporated herein by reference. Phenol itself is the phenol used in the greatest volume for the formation of such phenolic resins, but resorcinol, alkyl substituted phenols such as cresols, xylenols, and p-tert-butylphenol and p-phenylphenol are used in substantial volume. In the past, the aldehyde used has been almost exclusively formaldehyde, but small amounts or acetaldehyde and furfuraldehyde have also been used. The condensation of a phenol with an aldehyde is typically an acid catalyzed reaction, often a hydrochloric acid catalyzed reaction, with a molar ratio of aldehyde to phenol less than 1.
Conventional novolak resins, prior to cure, have only moderate thermal stability and typically melt within a range of from about 90.degree. C. to 120.degree. C., dependent upon the composition of the resin and its molecular weight. There has been little effort to increase the stability of the thermoplastic novolak resins to high temperatures because high thermal stability had not been considered to be an important property of a film forming resin nor had it been considered important in the formation of photoresist films. However, recent developments created a need for novolak resins having a greater melt temperature than those of the conventional novolak resins.
An effort to increase thermal stability of the novolak resins comprised the acid condensation of a mixture of a naphthol and a phenol with an aldehyde as disclosed in U.S. Pat. No. 4,424,315. These resins are copolymers formed by the aforesaid acid condensation of an aldehyde with an aromatic alcohol mixture of a naphthol and a phenol in the presence of an acid catalyst. The molar ratio of the naphthol to the phenol can vary from about 20 to 1 to 1 to 20 dependent upon the desired properties of the resin. These resins were prepared for use as a binder for a photoresist and showed improved resistance to flow at elevated temperatures, though it was found that photoresists formulated with such resins were difficult to develop.
In Materials for Microlithography, L. F. Thompson. G. G. Wilson, and J. M. Frechet; Eds.; ACS Symposium Series 266, American Chemical Society, Washington, D.C., 1984, Chapter 17, page 339, an m-cresol-benzaldehyde novolak resin was formulated with a photosensitizer and solvent to produce positive tone images when the mixture was applied to a silicon wafer, exposed to actinic radiation and subsequently developed. However, the synthesis of the cresol-benzaldehyde novolak resin, as taught, produced a material having low molecular weight, and photoresist compositions using this resin had low photospeed, low resolution and inadequate temperature resistance.
In U.S. Pat. No. 4,943,511, a positive photoresist composition is disclosed which uses a resin binder that is prepared from a phenolic component having a high p-cresol content and an aldehyde that is a mixture of formaldehyde and an aromatic aldehyde. In accordance with the patent, photoresists formulated using the aforesaid resins as binders possess improved resolution capabilities, but it is believed that the resins of the patent exhibit only minimal thermal improvement compared to prior art novolak resins.
U.S. Pat. No. 5,216,111 is directed to new resins comprising the condensation product of a phenol and an aromatic aldehyde, mixtures of such resin with other phenolic resins including conventional novolak resins--i.e., those prepared by the reaction of a phenol with formaldehyde in the presence of an acid catalyst, and to a method for the formation of said aromatic novolak resins. The resins disclosed in the patent exhibit glass transition temperatures in excess of 125.degree. C. and many exhibit glass transition temperatures as high as 175.degree. C. or higher. The resins are disclosed to be suitable for use in various coating compositions such as photoresist compositions and as precursors to heat resistant epoxy resins for use as laminate materials. Moreover, they are compatible with conventional novolak resins and other resins including other phenolic resins to provide new polymer mixtures exhibiting excellent film forming and thermal properties. Where the glass transition temperature of the resins disclosed and other resins used in combination with those of the invention are known, resin blends are readily prepared exhibiting any desired intermediate glass transition temperature by adjustment of the concentration of each resin in the blend.
An alternative approach to the formation of aromatic novolak resins is disclosed in U.S. Pat. No. 5,238,776 where the resin is the product resulting from the acid condensation of a bishydroxymethyl phenol with another phenol in the absence of an aldehyde. By the process of this patent, high molecular weight resins are formed having excellent thermal stability.
In each of the above process for the manufacture of phenolic resin, following the polymerization reaction, the phenolic resin is precipitated in water and removed from the reaction mixture by filtration. In the conventional process where a resin of high purity is required, such as in the manufacture of photoresists, the resin would then be dissolved in an organic solvent and passed through an ion exchange material to remove ionic contaminants. For example, it is known in the art that the acid condensation reaction used to form phenolic resins results in a resin containing acid residues. It is further known in the art that these acid residues are unacceptable contaminants in photoresists used for high resolution imaging such as in the fabrication of integrated circuit devices. Efforts to remove dissolved contaminants from materials used for the formation of photoresists by ion exchange are known in the art. For example, one such method is disclosed in International Publication No. WO 93/12152 which is directed to removing metal ions such as sodium and iron from novolak resins during their manufacture. The process comprises cation exchange treatment whereby a cation exchange resin is first washed with a mineral acid solution to reduce the level of total sodium and iron ions in the exchange resin to preferably less than 100 ppb, passing a formaldehyde reactant through the so treated cation exchange resin to decrease the sodium and iron ion content to less than 40 ppb, passing a phenolic compound through the cation exchange resin to decrease its sodium and iron ion content to less than 30 ppb, and then condensing the so treated phenolic compound with formaldehyde in the presence of an acid catalyst to form the resin.
A method for removal of dissolved ionic metals and non-metals from a photoresist is disclosed in Japanese Patent Appln. No.1228560, published Sep. 12, 1989. In accordance with the procedures of this patent, a photosensitive resin is passed through a mixed bed of strong cation exchange resin and an anion exchange resin to simultaneously remove cationic and anionic species from the photoresist solution.
Methods used to remove ionic contaminants in the prior art have not been completely satisfactory. For example, the use of strong acid ion exchange resins has been known to cause reaction of acid labile materials often found in photoresist products causing formation of undesirable impurities thus changing the concentration of the components in the photoresist composition. Moreover, the ion exchange materials used are costly and must be frequently replenished to remove exchanged contaminants. Finally, such materials often introduce acid into the material treated rather than remove the same.